The electric accumulator industry, which manufactures batteries for automobiles or other, stationary purposes, employs special machines for spreading lead oxide paste on lead alloy grids, which are thereafter usually known as plates.
The prior art teaches two basic processes for obtaining plates, which can be summarized as follows:
a) A first system is where the grids are transported on a conveyor belt below a hopper having spreading rollers.
The conveyor belt draws the grids across the rollers. To overcome the resistance produced by the lead oxide paste, the belt has to be soft and must exhibit a corrugated surface. Consequently and disadvantageously, it is difficult for the paste to reach the bottom surface of the grid on the conveyor belt since that surface is partially sunk in the soft surface of the belt, so that said bottom surface is at best only partially covered.
If a rigid belt is used instead, the above drawback is obviated, but the grids tend to slip and slide on the belt surface as they pass under the hopper rollers.
b) A second system provides two counter-rotating drawing rollers which push the grids under the hopper. After lead oxide application, the grids pass through a tunnel comprising one plate superposed on and parallel to another plate, which tunnel finishes the grid by skimming off any superfluous paste and leaving a correct thickness of paste on the grid.
Due to the considerable friction forces created in the tunnel area, a following grid is used to push a leading grid through. This leads to frequent blockage in the tunnel, which by necessity has to afford only a small passage space. Add to this the low mechanical resistance of the grid plates, which are only from 1.2 mm to 1.6 mm thick, and bearing in mind that they are in fact grids and not strong, and it will be easy to imagine how they deform and suffer from such treatment.
Furthermore, even leaving aside the above drawback, the final thickness of the lead oxide paste covering is not optimally controllable.